This invention generally relates to a method and apparatus for bonding films of resinous material to one another and to the article formed thereby. More particularly, the invention relates to bonding polyester films.
Films of resinous material such as polyester films have been available for many years, and they have been put to numerous uses. One particular application for such films of resinous materials is in the manufacture of bags or packages, for example, for the food packaging industry where packages are continually being designed to lessen the work of housewives. In particular, frozen or refrigerated food products which are generally heated or cooked before being served can be packaged in bags formed from films of resinous materials, for example, polyester films. Such bags or packages with their contents can be placed directly into a warm oven or in a pot of boiling water so that the foods can be heated or cooked before serving. Polyester films are particularly desirable for such applications in that they are transparent, durable, and chemically inert, and they have outstanding barrier properties.
In order to form a bag or package from polyester films, the films must be bonded together or sealed in some manner. One method of sealing polyester films is to coat the films with a readily heat-sealable material such as polyethylene in the area of the bond and then heat-seal the coatings to one another. However, bonds formed in this manner are generally incapable of withstanding elevated temperatures, for example, the temperature required to cook a roast in an oven. Another method has been to employ a cement material to bond the polyester films together, but such methods present difficult production problems. One other known method for bonding polyester films is to apply heat directly to the polyester films to be heat-sealed. However, this latter method will not always form a bond between the polyester films, particularly when the bond is formed under conditions which would permit the bond to be used for commercial applications. Further, when a bond is formed between the polyester films the areas of the films adjacent the bond generally deteriorate. Thus, even if the bond has sufficient strength characteristics, the areas of the polyester films around the bond will not have sufficient strength for many commercial applications.
Accordingly, difficulty has heretofore been experienced in forming a satisfactory seal directly between films of resinous material such as polyester films, and various methods have been substituted which employ an intermediate sealing material. Consequently, there is no known method available for directly bonding polyester films to one another which has extensive commercial applications. In particular, known methods for bonding polyester films do not form a seal capable of withstanding elevated temperatures and which has sufficient strength for many of the most advantageous applications of the film.